Furnace apparatus for blocking sodium ions

ABSTRACT

The present invention relates to a processing apparatus for growing and annealing a silicon dioxide insulator layer to obtain a mobile positive ion free silicon dioxide insulator layer. A mobile positive ion free silicon dioxide insulator layer is required in order to make a stable insulated gate field effect transistor. The processing apparatus comprises a nonoxidizing high-melting-point platinum metal film coated quartz furnace tube, potential means for placing a positive potential upon the platinum metal film coating of the platinum metal film coated quartz furnace tube to repel mobile ions therefrom, heater means for heating the interior of the quartz furnace tube, and gas means for passing oxygen gas through the platinum metal film coated quartz furnace tube. A silicon wafer may be oxidized in said processing apparatus to form a relatively mobile positive ion free silicon dioxide insulator layer of an insulated gate field effect transistor upon the silicon wafer. The silicon dioxide insulator layer is relatively uncontaminated by mobile positive ions which exist to the outside of the platinum metal film coated quartz furnace tube. A silicon wafer which previously has been coated by a silicon dioxide insulator layer may be processed to remove mobile ions within the silicon dioxide insulator layer. Mobile positive ions are repelled by the positive potential applied to the platinum metal film away from the outside of the quartz furnace tube, and mobile positive ions of the silicon dioxide insulator layer within the platinum metal film coated quartz tube are removed by the flowing oxygen gas due to the low-vapor pressure of the mobile positive ions.

United States Patent Koepp et a1.

[54] FURNACE APPARATUS FOR BLOCKING SODIUM IONS [72] Inventors: RonaldL. Koepp, Dayton; Stanley J. Dudkowski, Kettering, both of Ohio [73]Assignee: The National Cash Register Company,

Dayton, Ohio [22] Filed: Oct. 14, 1969 1211 Appl. No.: 866,185

[52] US. Cl. ..23/252 R, 13/1, 13/20, 23/277, 117/95, 117/107, 117/227,117/229,

[51] Int. Cl. ..B0lj l/00,C23c 11/00 [58] Field ofSearch ..23/252, 277,288 J, 288 M; 118/48, 49, 49.1, 49.5; 117/95, 229, 107 US, 227,

Primary ExaminerJoseph Scovronek Att0rneyLouis A. Kline, John .1.Callahan and John P. Tarlano 1 Feb. 29, 1972 [57] ABSTRACT The presentinvention relates to a processing apparatus for growing and annealing asilicon dioxide insulator layer to obtain a mobile positive ion freesilicon dioxide insulator layer, A mobile positive ion free silicondioxide insulator layer is required in order to make a stable insulatedgate field effect transistor. The processing apparatus comprises anonoxidizing high-melting-point platinum metal film coated quartz fumucetube, potential means for placing a positive potential upon the platinummetal film coating of the platinum metal film coated quartz furnace tubeto repel mobile ions therefrom, heater means for heating the interior ofthe quartz furnace tube, and gas means for passing oxygen gas throughthe platinum metal film coated quartz furnace tube. A silicon wafer maybe oxidized in said processing apparatus to form a relatively mobilepositive ion free silicon dioxide insulator layer of an insulated gatefield effect transistor upon the silicon wafer. The silicon dioxideinsulator layer is relatively uncontaminated by mobile positive ionswhich exist to the outside of the platinum metal film coated quartzfurnace tube. A silicon wafer which previously has been coated by asilicon dioxide insulator layer may be processed to remove mobile ionswithin the silicon dioxide insulator layer. Mobile positive ions arerepelled by the positive potential applied to the platinum metal filmaway from the outside of the quartz furnace tube, and mobile positiveions of the silicon dioxide insulator layer within the platinum metalfilm coated quartz tube are removed by the flowing oxygen gas due to thelow-vapor pressure of the mobile positive ions.

4 Claims, 5 Drawing Figures FURNACE APPARATUS FOR BLOCKING SODIUM IONSBACKGROUND OF THE INVENTION U.S. Pat. No. 3,380,852, issued Apr. 30,1968, on the application of Adolf Goetzberger, discloses a method offorming a relatively uncontaminated silicon oxide layer upon a siliconwafer, comprising placing a silicon wafer within a quartz furnace tube,placing a wire electrode in close spatial relation above the siliconwafer, applying'a positive potential to the electrode with respect tothe silicon wafer, and oxidizing the silicon wafer with the positivepotential applied to the electrode. Goetzberger states that charges fromthe positively charged electrode neutralize negative ions within thesilicon dioxide layer, so as to form a relatively uncontaminated silicondioxide layer.

'Goetzberger does not disclose a metal film coated quartz furnace tubehaving a positive potential applied to the metal film to repel positiveions away from the exterior of the metal film and to allow a flowingoxidation gas to absorb positive ions from within a silicon dioxidelayer. Goetzberger has only a penetrable positive potential around asilicon wafer, by setting up an electric field between a positivelycharged wire electrode and the negatively charged wafer.

In the processing apparatus of the present invention, a potentialdifference is not set up between the metal film coating of a metal filmcoated quartz furnace tube and a silicon wafer, but the silicon waferexists within a positive equipotential closed metal film. The siliconwafer is not subjected to an electric field during its oxidation orduring its annealing, but both during oxidation and during annealing,positive ions that may exist to the outside of the metal film coatedquartz furnace tube are repelled and prevented from entering the metalfilm coated quartz furnace tube, as a result of a positive potentialexisting upon the metal film coating of the metal film coated quartzfurnace tube with respect to ground. Therefore positive ions areprevented from diffusing through the metal coated quartz furnace tubeand combining with a silicon dioxide insulator layer during itsformation upon the silicon wafer. Positive ions are also prevented fromdiffusing through the metal film coated quartz furnace tube to combinewith a silicon dioxide insulator layer during its annealing. Duringannealing of the silicon dioxide insulator layer, positive ions emittedfrom the silicon dioxide insulator layer are absorbed in a flowingoxidation gas which is passed through the interior of the metal filmcoated quartz furnace tube.

The positively charged wire electrode of Goetzberger repels somepositive ions that exist on the outside of the area between thepositively charged wire electrode and the negatively charged siliconwafer, but does not repel positive ions which get into said area. Infact, positive ions which get into said area are absorbed by thenegatively charged silicon wafer.

The processing apparatus of the present invention hinders thecombination of positive ions which may enter within themetal-film-coated quartz furnace tube with a silicon dioxide insulatorlayer therein, due to the flushing of the metal-filmcoated quartzfurnace tube with oxygen gas.

SUMMARY OF THE INVENTION DESCRIPTION OF THE DRAWING FIG. 1 is aperspective view of the processing apparatus of the present inventionused as an annealing furnace.

FIG. 2 is a cross-sectional view of an externally coate processingfurnace tube.

processing apparatus of the present invention used as an oxidationfurnace.

FIG. 5 is a cross-sectional view of an MOS field effect transistorhaving a positive sodium ion free silicon dioxide insulator layer.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 shows a processingapparatus for annealing a silicon dioxide insulator layer to deplete theconcentration of positive ions therein. As shown in FIG. 1, a0.5-centimeter-thick furnace tube 12 has evaporated upon its innersurface 13 a 700- Angstrom-thick nonoxidizing, high-melting-point metalfilm 14 to form an inner wall therein. The metal film 14 is preferably aplatinum or rhodium film. However, a tantalum or titanium film, with anoxygen-impervious silicon nitride layer thereon, can be used. The metalfilm 14, even through being an evaporated amorphous film on the furnacetube I2, is a good barrier to the passage of mobile ions, such as mobilepositive sodium ions, due to its crystal structure.

In accordance with the present invention, a metal film 14 has been foundto be necessary to make a resistively heated furnace tube, such as aquartz, aluminum oxide, silicon carbide, or silicon nitride furnacetube, impervious to ions, such as positive sodium ions, at hightemperature. A resistively heated furnace tube 12 will reach atemperature of approximately 1,100 C., at which temperature it is porousto positive sodium ions. That is, sodium ions are highly mobile indiffusing through the crystalline structure of the furnace tube 12.

A platinum film 14 should be as thick as 600 Angstroms to stop most ofthe mobile ions from getting therethrough. However, a thinner film wouldbe fairly effective in stopping mobile ions. An evaporated platinum film14 which is made thicker than 600 Angstroms is increasingly effective instopping mobile ions. An evaporated platinum film 14 which is thickerthan 6,000 Angstroms begins to peel from the quartz furnace tube 12.Therefore, a platinum film 14 which is between 600 and 6,000 Angstromsthick is preferred.

As shown in FIG. 2, a platinum film 8 may be evaporated on the outsideof a quartz furnace tube 6. Such a platinum film 8 which is between 600and 6,000 Angstroms thick is effective in stopping mobile ions fromentering the interior of the quartz furnace tube 6. However, sincemobile ions are located in the quartz furnace tube 6 itself, it ispreferable that a platinum film be on the inside of the quartz furnacetube 6.

In FIG. 1, the platinum film 14, being on the inner surface of thefumace tube 12, hinders the mobile positive sodium ions within the wallof the furnace tube itself from diffusing into the interior of thefurnace tube 12 at high temperature. The platinum film 14 does not meltat 1,100 C. and does not oxidize at 1,100 C. in an oxidizing atmosphere.Gold, which also is a metal, melts near 1,063 C. Therefore, gold cannotbe used to coat a quartz furnace tube 12 which is heated to 1,100 C.Silver also melts below 1,100 C. A copper film melts below 1,l00 C.Therefore a copper film may not be used to form a nonporous barrier tothe passage of mobile ions into the interior of the quartz processingfurnace tube 12 when it is used in a high-temperature oxidationprocessing apparatus.

As shown in FIG. 3, a tantalum film 10 may be evaporated upon the insidesurface of a quartz furnace tube 9. The tantalum film l0 oxidizes in anoxidizing atmosphere, at 1,l00 C. However, a layer 11 impervious tooxygen at high temperature, such as a silicon nitride layer, is placedupon the exposed surface of the 700-Angstrom-thick tantalum film 10. Thesilicon nitride layer 11 prevents oxygen in the interior of thetantalum-coated quartz furnace tube 9 from oxidizing the tantalum film10. The tantalum film 10 on the inner surface of the quartz furnace tube9 prevents mobile ions from entering the interior of the quartz furnacetube 9.

A metal film is preferred for use in the processing apparatus of thepresent invention which has no mobile ions, has a highmelting point, anddoes not oxidize at a high temperature. A platinum film is a very goodmetal film which has these properties.,Its atomic crystalline structureis fine enough to stop mobile ions from passing through it at hightemperature.

As shown in FIG. 1, the platinum film 14, having been evaporated, is anamorphous platinum film. The evaporation causes the platinum film, whichis evaporated on the inside of a quartz tube, to be amorphous. However,the evaporated film 14, being semicrystalline, is impervious to mobileions.

It is to be observed that a solid metal furnace tube, such as a platinumfurnace tube, can be used in place of a metal-filmcoated furnace tube. Asolid platinum furnace tube is impervious to mobile ions, such as mobilepositive sodium ions, at a temperature of 1,100" C. The inner wall of asolid platinum furnace tube is, of course, composed of a metal.

As shown in FIG. 1, a battery 18 is attached to the end of the platinumfilm 14 by means of a lead 16. The edge of the platinum film 14 willreach a temperature of only about 100 C., since it is notdirectly underthe radiant heating coil 30. The battery 18 applied a positive 500 voltspotential, with respect to ground, to the 700-Angstrom-thick platinumfilm 14. By applying a positive potential to the platinum film 14 on thefurnace tube 12, one can further hinder the passage of mobile positivesodium ions from the outside of the tube 12 into the interior of thefurnace tube 12.

A silicon wafer holder 22 is laid within the platinum coated quartzfurnace tube 12. A silicon wafer 24, havinga 1,200- Angstrom-thicksilicon oxide insulator layer 26, is held by the silicon wafer holder 22within the platinum coated quartz furnace tube 12. The resistanceheating coil 30 is placed around the outside of the platinum-coatedquartz furnace tube 12.

The platinum film 14 is held at a positive 500 volts with respect toground. Heat from the' radiant heating coil 30, which is driven by an ACpower source 34, raises the temperature of the silicon wafer 24 withinthe center of the platinumcoated quartz furnace tube to 600 C. A 100cc./minute flowing oxygen gas from an oxygen container is passed through95 C. water 32 within a container 33, and then through theplatinum-coated quartz furnace tube 12 while it is being heated, for 60minutes. In accordance with the present invention, a 14-fold reductionin the number of mobile positive ions, including mobile positive sodiumions, within the silicon oxide insulator layer 26 is achieved, using apositively charged platinum-coated quartz furnace tube 12, over what canbe obtained using an uncoated quartz furnace tube. A threefold reductionin the number of mobile positive ions, including mobile positive sodiumions, is achieved with the use of a positively charged platinum-coatedquartz furnace tube 12, over what can be obtained with the use of anuncharged platinumcoated quartz furnace tube 12..

FIG. 4 shows a processing apparatus for oxidizing semiconductor materialin a mobile-positive-ion-free furnace tube 40. As shown in FIG. 4, aplatinum-coated quartz furnace tube 40 is positioned by a suitable meanswithin a resistance heating coil 42. A 700-Angstrom-thick platinum film44 is evaporated upon the inside surface 45 of the quartz furnace tube40. Oxygen gas is pased through the quartz furnace tube 40 from anoxygen container 48 at the rate of 300 cc./minute. Radiant heat, fromthe resistance heating coil 42, which is driven by an AC power source50, raises the temperature inside the quartz furnace tube 40 to l,l C. Apositive potential of +500 volts from a battery 60 is attached to theplatinum film 44 by means of a lead 62, with respect to ground. Theoxygen gas in the platinum-coated quartz furnace tube 40 grows arelatively mobile-positive-ion-free silicon oxide insulator layer 70 ona silicon wafer 72, which is placed on a silicon wafer holder 74 in theplatinum-coated quartz furnace tube 40. An exit port 80 allows theoxygen gas to exit from the quartz furnace tube 40. An MOO-Angstromsilicon oxide insulator layer 70 is grown upon an N-type'silicon wafer72, having P-type source and drain regions diffused therein, byoxidizing it for 180 minutes.

It is found that the silicon oxide insulator layer 70 which is producedhas a IO-fold reduction in the concentration of mobile positivelycharged ions therein over a silicon dioxide insu lator layer that isproduced in a quartz furnace tube which does not have a platinum filmthereon. A twofold reduction in the concentration of mobile positiveions is produced therein from a silicon dioxide insulator layer producedin a platinum coated quartz furnace tube 40 which does not have apositive potential applied to the platinum film 44.

FIG. 5 shows an MOS field effect transistor having amobile-positive-ion-free silicon dioxide insulator layer 70 therein. Asshown in FIG. 5, the silicon dioxide layer 70 is selectively etched soas only to extend from the edge of the P- type source region 99 to theedge of the P-type drain region 92.

A small area of a partially processed silicon wafer 72 can then befabricated into a completed metal-silicon oxide-silicon (MOS) fieldeffect transistor 100, as shown in FIG. 5. A gate electrode 102, such asan aluminum gate electrode, is deposited by vacuum evaporation upon thesilicon oxide layer 70 through an evaporation mask. A source electrode103 and a drain electrode 105 are attached to the P-type source region99 and to the P-type drain region 92, also by vacuum deposition. Ametal-silicon oxide-silicon (MOS) field effect transistor is therebyproduced.

Due to the formation of the silicon oxide insulator layer 70 within amobile-positive-sodium-ion free environment, mobile positive sodium ionsare not trapped within the silicon dioxide insulator layer 70 of the MOSfield effect transistor 100. The

mobile-positive-sodium-ion-free silicon dioxide layer 70, therefore,aids in producing an MOS field effect transistor 100, which begins toconduct a source-drain current, at a small 3 volts threshold voltagefrom a.,battery 110. The amount of source-drain current from the battery112 also does not appreciably drift for a given gate voltage, under theperiodic operation of the MOS field effect transistor 100. That is, theprocessing apparatusof the present invention aids in producing a silicondioxide insulator layer 70, upon a silicon wafer 72, in such a way as toretard the trapping of mobile positive sodium ions within the silicondioxide insulator layer 70 of the MOS field effect transistor [00. Thedecreased drift in the source-drain current for a given gate voltage, inthe MOS field effect transistor 100, is due to the near lack of mobilepositive' sodium ions in the silicon dioxide insulator layer 70. Ifsodium atoms couldmigrate within the silicon dioxide insulator layer 70,a greater negative threshold voltage than 3 volts would be required, theadded gate voltage being necessary to make up for the chargeconcentration of positive sodium atoms in the silicon dioxide insulatorlayer 70.

What is claimed is: A t

1. An impervious processing furnace tube whose inner wall surface iscoated with an amorphous nonoxidizing high-melting-point nonporous metalfilm of thickness between approximately 600 and 6,000 Angstroms, whichis a nonporous barrier to the passage of mobile positive sodium ionsinto the interior of the processing furnace tube between a temperatureof approximately 600 C. and 1,200 C., the impervious processing furnacetube having a wall thickness of approximately 0.5 centimeters. i I

2. An impervious quartz processing furnace tube composed of a processingfurnace tube whose inner wall surface is successively coated with ahigh-melting-point nonporous metal filrn of a thickness betweenapproximately 600 and 6,000 Angstroms and a silicon nitrideoxygen-impervious layer to prevent oxidation of said metal film, themetal film being a nonporous barrier to the passage of mobile positivesodium ions into the interior of the processing furnace tube between atemperature of approximately 600 C. and 1,200 C., theimpervious-processingfurnace tube having a wall thickness ofapproximately.0.5 centimeters.

3. A processing. apparatus for processing semiconductor material in amobile-sodium-ion-free furnace tube comprising:

material in a mobile-sodium-ion-free furnace tube, comprising:

an impervious processing furnace tube having a wall thickness ofapproximately 0.5 centimeters and a thin nonoxidizing,high-melting-point nonporous metal film thickness between approximately600 and 6,000 Angstroms coated upon said furnace tube to shield theinterior of said tube from mobile sodium ions between a temperature ofapproximately 600 C. and [200 C.;

positive potential means connected to said metal film to place apositive potential thereon for further hindering mobile positive sodiumions from entering the interior of said furnace tube; and

gas means connected to said tube for passing a gas through said furnacetube to flush mobile ions from the interior of said furnace tube.

2. An impervious quartz processing furnace tube composed of a processingfurnace tube whose inner wall surface is successively coated with ahigh-melting-point nonporous metal film of a thickness betweenapproximately 600 and 6,000 Angstroms and a silicon nitrideoxygen-impervious layer to prevent oxidation of said metal film, themetal film being a nonporous barrier to the passage of mobile positivesodium ions into the interior of the processing furnace tube between atemperature of approximately 600* C. and 1,200* C., the imperviousprocessing furnace tube having a wall thickness of approximately 0.5centimeters.
 3. A processing apparatus for processing semiconductormaterial in a mobile-sodium-ion-free furnace tube comprising: animpervious processing furnace tube having a wall thickness ofapproximately 0.5 centimeters and a thin nonoxidizing,high-melting-point nonporous metal film of a thickness betweenapproximately 600 and 6,000 Angstroms coated upon said furnace tube, toshield the interior of said furnace tube from mobile sodium ions betweena temperaturE of approximately 600* C. and 1,200* C., and a positivepotential means connected to said metal film to place a positivepotential thereon for further hindering mobile positive sodium ions fromentering the interior of said furnace tube.
 4. A processing apparatusfor processing semiconductor material in a mobile-sodium-ion-freefurnace tube, comprising: an impervious processing furnace tube having awall thickness of approximately 0.5 centimeters and a thin nonoxidizing,high-melting-point nonporous metal film thickness between approximately600 and 6,000 Angstroms coated upon said furnace tube to shield theinterior of said tube from mobile sodium ions between a temperature ofapproximately 600* C. and 1,200* C.; positive potential means connectedto said metal film to place a positive potential thereon for furtherhindering mobile positive sodium ions from entering the interior of saidfurnace tube; and gas means connected to said tube for passing a gasthrough said furnace tube to flush mobile ions from the interior of saidfurnace tube.